The present invention relates to an electric synchronous machine which is excited via rotating rectifiers from the rotating, Y-connected three-phase winding of an exciter machine, for which purpose the outputs of the three-phase winding are coupled to the diodes of a Y-connected rectifier, the Y-point of which is connected to one end of the exciter winding of the synchronous machine, while the other end of the exciter winding is connected to the central point of the three-phase winding, and wherein auxiliary slip rings are provided on the machine shaft, one of which is connected to the central point of the three-phase winding, the slip rings engaging brushes that can be lifted off, the slip rings coupling the brushes via switches to stationary electrical elements.
Apparatus for exciting a synchronous machine via rotating rectifiers with the exciter winding connected to the center of the three phase winding is known from U.S. Pat. No. 3,303,410. The known synchronous machine is a synchronous generator, the rotating excitation circuit of which is not in operation while the synchronous machine is starting up asynchronously. Although the excitation of the synchronous generator via the rotating rectifier is designed without slip rings, two auxiliary slip rings are further provided on the shaft of the known synchronous machine, which permit connection to stationary electrical elements via brushes that can be lifted off and switches. This circuitry, however, relates to a device for detecting shorts to ground in the rotor which is connected only temporarily and thus does not influence the operation of the synchronous generator which otherwise has no slip rings.
While in synchronous generators excited without slip rings, the excitation circuit is not operative during the asynchronous starting of the machine, the latter constitutes normal operation for synchronous motors excited without slip rings. In this connection, it is known, for instance, from German Pat. No. 15 63 329 and DE-OS 32 18 033, to excite the synchronous motor by means of an external-pole synchronous exciter machine without slip rings by letting the rotating three-phase winding of the external-pole synchronous exciter machine fed via a rotating rectifier connected in the three-phase bridge circuit feed the exciter winding of the synchronous motor. To this end, a series circuit of two thyristors poled in the forward direction of the field current is connected parallel to the exciter winding, the connecting line of which is connected to one lead of the three-phase winding. In addition, a firing device which fires in dependence on the voltage is connected between the positive pole of the field winding and the control electrodes of the thyristors, so that the negative half-waves of the alternating field current flow through the thyristors and the excitation circuit is protected against overvoltages. These firing devices are operative up to the time of the synchronization of the synchronous motor.
It is further known that several converter sets, each comprising a synchronous generator driven by a synchronous motor, can operate in parallel only if all drive motors are operated with the same pole wheel position, since otherwise the synchronous generators to be switched cannot be synchronized. Therefore, it is necessary to be able to correct the pole wheel position in such synchronous/synchronous converters subsequently by letting the drive motor slip in a specificed manner, the drive motors of which reach the synchronous speed always without excitation and in general without the correct pole position relative to each other because a counter-torque is lacking during the unloaded asynchronous start up.